In the treatment of tumors, autoimmune diseases, allergies and tissue rejection reactions, it is a disadvantage that the currently available medicaments, such as chemotherapeutic agents (Goodin, S., Am J Health Syst Pharm, 2007, 64: p. 15-24) corticosteroids (Stanbury, R. M. & Graham, E. M., Br J Ophthalmol, 1998, 82: p. 704-708) and immunosuppressive agents (Stucker, F. & Ackermann, D., Ther Umsch, 2011, 68: p. 679-686), have a potential of side effects which is sometimes considerable, due to their relative non-specificity. It has been attempted to moderate this by various therapeutical concepts. Especially the use of immunotherapeutic agents is an approach, which resulted in an increase of the specificity of medicaments, especially in tumor treatment.
Immunotoxins (ITs) are proteins originally developed for the treatment of malignant diseases. They comprise a toxic effector domain and a tumor cell-specific binding component, which is usually an antibody or a derivative thereof. Initially the antibody components were derived from mice and the toxins were derived from bacteria or plants, e.g. Pseudomonas aeruginosa exotoxin A (ETA) (Ribbert, T., et al., Br J Dermatol, 2010; 163: p. 279-86), diphtheria toxin (DT) (Potala, S., et al., Drug discovery today, 2008, 13: p. 807-15) or ricin A toxin (RT) (Thepen T., et al., Nature biotechnology, 2000, 18: p. 48-51.). The major advantage of ITs compared to traditional chemotherapy is their exceptional specificity towards targeted cells.
Fusion proteins that consist of a disease-specific binding component (e.g. scFvs, cytokines or peptide ligands) fused to a human toxic effector domain like a pro-apoptotic enzyme are known as ‘human cytolytic fusion proteins’ (hCFPs) and have already proven their potential in several applications (Stahnke, B., et al., Mol Cancer Ther, 2008. 7(9): p. 2924-32; Huhn, M., et al., Cancer Res, 2001. 61(24): p. 8737-42; Krauss, J., et al., Br J Haematol, 2005. 128(5): p. 602-9; Schiffer, S., et al., Antibodies, 2013. 2(1): p. 9-18; Hristodorov, D., et al., Br J Cancer, 2014. 109(6): p. 1570-1578).
The therapeutic activity of most anticancer drugs in clinical use is limited by their general toxicity to proliferating cells, including some normal cells. Although, chemists continue to develop novel cytotoxic agents with unique mechanisms of action, many of these compounds still lack tumor selectivity and have not been therapeutically useful. Antibodies or fragments ablated thereof meaning also ITs and hCFPs bind to specific markers on the surface of cancer cells offer an alternative therapy that is tumor highly specific and thus much less toxic (Chari, R V, Accounts of chemical research, 2008, 41: p. 98-107).
However, in order to obtain the desired cytotoxic effect of ITs and/or hCFPs some processing steps in the targeted cells are crucial. Next to the necessary binding and internalization of the IT- or hCFP-antigen complex, key impact on the activity has the subsequent intracellular routing with and cytosolic delivery Berges, N., et al. Antibodies, 2014, 3(1): p. 92-115).
An example is the treatment of leukaemia using immunotherapy, which requires the targeting of specific antigens on the surface of blasts. The Fc gamma receptor (FcγRI, CD64) has been investigated into details. The CD64-targeting immunotherapy has shown promising efficacy in the targeted ablation of acute myeloid leukaemia (AML) and chronic myeloid leukaemia (CML) cells. Thus far, CD64 is the only Fc receptor deemed suitable as an immunotherapeutic target for both oncogenic and inflammatory diseases. Nevertheless, there are plenty of clinical cases of malignancies lacking the CD64 expression (Schiffer, S., et al., Int J Cancer, 2014, 135: p. 1497-508), which excludes the possibility of efficient CD64-targeting therapy.
Furthermore, the cytotoxic efficacy of the IT- or hCFP is crucially dependent on the used scFv and the subsequent intracellular routing occurring after the IT- or hCFP-antigen complex internalization. Thus, the chosen scFv, in combination with the toxic effector domain, has to fulfill several requirements in order to acquire IT- or CFP cytotoxicity.
As a result thereof, the availability of novel ITs or human cytolytic fusion proteins comprising new binding moiety, which covers all requirements, would be highly advantageous.